Fixing a laboratory waterbath

[eccerr0r]

what's bothering me now is I don't see a rectifier and how it derives the negative voltage for the 7107...
and the track wire mess with all those jumper connections is causing my brain to hurt... hmm....

[vrasp]

Is there any test I can do to help figure this out?

What if I ignore the 20v winding and just hook up 8v and 15v and see if it works? Although I m also curious to know why there is a 20v winding if there is no use for it.

[eccerr0r]

It is being used, I just don't quite understand how it's being used... You will need all three but I don't understand the 20V yet -- if it indeed is 20V. It could be the 15 still... Or even 8...

To be honest the voltages in the order of the pin outputs is also possible though it'd be dissipating more heat, so the 15VAC -> 5VDC, 20VAC -> 12VDC, and the 8V -> display board...

[vrasp]

Looking at the picture of the transformer:



Is there a chance is still works? Maybe I can hook it up to 120v and measure the voltage on the secondary to have a better idea?

EDIT: Sorry, not sure how to resize that image so it's no huge.

[petehall347]

aeg b40c are bridge rectifiers

[petehall347]

the last pic shows only 4 wires where there should be 6 .

[vrasp]

Originally Posted by petehall347

the last pic shows only 4 wires where there should be 6 .

Yes that got me confused as well. Now that I look at it, I might have inserted those wires between where the pins were. You can see 6 small incisions in between. I can see if that is it.

[eccerr0r]

I think he stuck desoider braid in the slots between the wire connections of the transformer... The remainder of the pin stubs are within the plastic to the two sides of where the desolder braids are shoved in.

[vrasp]

I don't think I can do anything with this transformer.

I dont know if this helps, but I read on the 7812CT series datasheet that it can take up to 30v on the input. Maybe this line is 20v?

https://datasheetspdf.com/pdf/540243/Vishay/MC7812CT/1

[eccerr0r]

Yes technically you can go up higher but the higher the input, the hotter the device will be. As far as I can tell, the two wire pairs that go straight to a bridge and regulator, limited by how hot the regulator will get, you can use pretty much any voltage. That third output pair... is still up in the air...

BTW if you want to go ahead with experimenting, of course at your expense that it could make things worse, you should locate:

9VAC-15VAC transformer to feed into the 5V supply line (my guess now is that the 15V line went here, since the manufacturer seems to have deliberately put the paint in this order...)
15VAC-20VAC transformer to feed the 12V supply line (same deal for the 20V line)
8VAC transformer to feed that third pair that goes to the display board. Actually, know what... this one is critical that it's 8V and not any higher perhaps, this is my current guess as this is the main LED supply. You probably can get away with using a common 6.3VAC transformer here though your display may be a bit dimmer.

[vrasp]

Could you tell me which is which again. I coloured them to make it easier.

Is it:

Blue: 15v
Red: 5v (rectified 8v from the transformer)
Green:
Yellow:

That would be really helpful.

[eccerr0r]

The board with the transformer on it is probably better to do wiring hookups perhaps?
DNT = Do Not Touch
8V = 8 volt output, I would say 6.3VAC-8VAC is good
15 = 15 volt output, I'd suggest anywhere from 9VAC to 15VAC
20,20V = 20 volt output, I'd suggest anywhere from 14VAC to 20VAC

Do not go higher than the suggestions in my opinion.

BTW
only the 8V output needs to go to 1A, the other two outputs probably 200-500mA is sufficient.

and still... how did that trace on the transformer board burn out? Fed it 220V without changing from 120V or something?

[vrasp]

Originally Posted by eccerr0r

The board with the transformer on it is probably better to do wiring hookups perhaps?
DNT = Do Not Touch
8V = 8 volt output, I would say 6.3VAC-8VAC is good
15 = 15 volt output, I'd suggest anywhere from 9VAC to 15VAC
20,20V = 20 volt output, I'd suggest anywhere from 14VAC to 20VAC

Do not go higher than the suggestions in my opinion.

BTW
only the 8V output needs to go to 1A, the other two outputs probably 200-500mA is sufficient.

Yes, I was going to do the hook ups on the transformer board.

When you say "DNT" you mean do not touch with bare hands? Or the pins shouldnt touch eachother?

Thanks for the diagram! Again, super helpful.

[eccerr0r]

Do Not Touch = Do not solder the transformer or anything else to them.

And technically you probably could get two (*maybe* just one) 5V and a 12V SMPS/regulated wall warts to replace the transformer, bridges, and 7805/7812; but there's a lot of surgery involved.

[vrasp]

Sounds good. I ve never bought a transformer before. I see lots of vocabulary I don't know. This is what I found. What do you think?

[eccerr0r]

The two higher voltage (20V/"15"V) units are much, much bigger than necessary. You will not even get close to 1A - the other components can't handle the current, otherwise the specifications are fairly typical. The only transformer that you could get close to 1A is the low voltage "8V" unit and if you reduce the voltage to 6.3V, you'll need even less current.

Main thing is that it needs to fit in the space you have...

[vrasp]

I think 2 will fit. Three.. well I think it can fit but I d have to figure out a way.

I changed the 20v and 15v units for these. They both have 300mA.

Voltage - Primary
115V
Voltage - Secondary (Full Load)
12V
Current - Output (Max)
300mA
Primary Winding(s)
Single
Secondary Winding(s)
Single
Center Tap
Yes
Power - Max
3.6VA
Mounting Type
Chassis Mount
Termination Style
Wire Leads
Size / Dimension
60.45mm L x 35.05mm W
Height - Seated (Max)
35.05mm

Voltage - Primary
115V
Voltage - Secondary (Full Load)
20V
Current - Output (Max)
300mA
Primary Winding(s)
Single
Secondary Winding(s)
Single
Center Tap
Yes
Power - Max
6VA
Mounting Type
Chassis Mount
Termination Style
Solder, Quick Connect
Size / Dimension
60.20mm L x 41.28mm W



Would it make things much more complicated if I got a transformer that has several secondary outputs? Like maybe one that can do 20v and 15v and then I ll get a second transformer for the 8v. (as an example)

[eccerr0r]

Multiple outputs are fine, as long as they are isolated, and can deal with the total power. The size of the metal core on the transformer is a very strong indication of maximum power the transformer can work with hence knowing the size of the original gives a good estimate of wattage needed for the new ones.

[vrasp]

The original one is 43 W x 36 L x 32 H (mm), or if you work in inches, 1.69 x 1.41 x 1.26.

[vrasp]

My bad, the CENTER is 30 x 26 x 32 mm.